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April 4, 2012 -- Radiation therapy may not be necessary for women with the luminal A subtype of breast cancer, especially if they are older than 60, according to study results presented at the American Association of Cancer Research (AACR) meeting held this week in Chicago.

A Canadian research team performed molecular subtyping on 304 tumor blocks from 769 women with cancer who had been enrolled in a randomized clinical trial in which they received hormone therapy treatment (tamoxifen) and whole-breast radiation therapy treatment or hormone therapy alone. Based on the immunohistochemistry results, the patients were classified into six categories, one of which was luminal A. Luminal A was defined as ER-positive, PR-positive, HER2-negative, and low Ki-67.

Patients were followed for 10 years, Dr. Fei-Fei Liu, staff radiation oncologist at Princess Margaret Hospital in Toronto, Ontario, and professor of radiation oncology at the University of Toronto, told attendees.

Women in the luminal A subgroup had the best outcome, with a 10-year risk for local relapse of 8% with hormone therapy alone versus 4.6% for the combined treatment. Women in this subgroup older than 60 had a local relapse rate of 4.3% and 6%, respectively, suggesting that radiotherapy treatments did not influence the long-term recurrence outcomes of this group of patients.

However, the addition of radiation therapy was beneficial for the other five breast cancer subgroups. In the luminal B subgroup, for example, women who had hormone treatment only had a recurrence rate of 16.1% compared with a rate of 3.9% who had the combined treatment.